In a color electrophotographic copier, for example, an image recorded in a document is separated into three different colors (i.e. red, green, and blue) and, then, light images each being associated with a respective one of the colors are individually developed by cyan, magenta, and yellow toner, which are complementary to those colors, or black toner. Such development, therefore, needs a plurality of developing units. However, constructing the developing units independently of each other and arranging them around an electrostatic latent image carrier would prohibitively increase the overall dimensions of the copier.
In the light of the above, there has been proposed a rotary multicolor developing apparatus which develops an electrostatic latent image by sequentially applying toner of different colors to the latent image. In this type of developing apparatus, a drum is rotatably disposed closely to a latent image carrier, and the interior of the rotatable drum is partitioned to define a plurality of circumferentially adjoining compartments, or developing chambers. Each of the developing chambers is provided with an opening for development, and developing means to constitute a developing unit for a particular color. Such developing units are sequentially transported in a rotational motion to a prescribed position close to a latent image carrier, (i.e., a developing position) so that toner of different colors are applied from the respective developing units to a latent image on the latent image carrier so as to develop it in multiple colors.
However, a prior art developing apparatus of the type described is operable with a so-called one-component developer only and not with a two-component developer which is capable of producing images with high quality and stability. Specifically, a developing apparatus of the kind using a two-component developer has to be constructed to sufficiently agitate and mix the developer in order to maintain toner density uniform and to promote good toner charging. This requisite cannot be met unless the developing units are furnished with various kinds of agitating mechanisms as well as substantial spaces for defining agitation paths, independently of each other. This renders each of the developing units prohibitively bulky, and applying such bulky developing units to a rotary developing apparatus is impracticable. If not impracticable, such would not make any significant contribution to the miniaturization of a developing apparatus in relation to the rotary construction.
In a rotary multicolor developing apparatus, the above-stated developing operation is usually effected with one of the developing units located in the predetermined developing position. Another problem with this type of prior art developing apparatus is that it is extremely difficult for each developing unit to be located in the developing position with accuracy. The resultant inaccurate distance between the latent image carrier and a developer carrier of the developing unit, (i.e., a gap for development) affects the quality of image reproduction to a critical degree.
In a rotary multicolor developing apparatus of the kind described, while one developing unit is brought to the developing position, a drive input portion of a developer carrier which is included in that developing unit is brought into connection with a drive output portion of a drive system that is mounted on the machine body, thereby transmitting a drive force for allowing the developer carrier to perform a predetermined motion for toner supply. In this regard, the prior art developing apparatus suffers from a drawback that the drive input and output portions (e.g., gears) cannot be smoothly brought into mesh with each other and generate impacts quite often. This stems from the fact that the drive output portion of the drive system (i.e., an output gear) is connected to a drive source at all times and, therefore, is provided with little freedom. The impacts frequently result in damage to various kinds of parts, generation of noise, and incomplete drive.
Still another problem with a rotary multicolor developing apparatus is that toner is often allowed to leak or scatter from both end portions of the opening for development, which do not contribute to development. The leaked or scattered part of toner deposited on optical parts critically impairs the optical characteristics of those parts, thereby deteriorating the quality of image reproduction. Deposition of such toner on electrical parts short-circuits them, resulting in malfunctions. Further, deposition of such toner on mechanical slidable parts accelerates the wear of those parts, to shortening their service life. Moreover, in the case of color development, the leaked or scattered part of one toner would be mixed with those of others, preventing different colors from being reproduced regularly.
In a prior art rotary multicolor developing apparatus, it has been customary to arrange, for example, a yellow developing unit, a magenta developing unit, a cyan developing unit, and a black developing unit in this order within and along the periphery of a rotary drum, so that toner of different colors may be applied to a latent image in the order of transparency, the lowest one first. In such a construction, when an image consisting of black characters and magenta (red) characters is to be recorded (as is desired most frequently), what is required is developing a latent image in black by the black developing unit, then skipping the cyan developing unit, and then transporting the magenta developing unit to a prescribed developing position. To implement ordinary black-and-white copying, which may be selected thereafter, the yellow developing unit has to be skipped after the completion of the development in magenta, followed by development in black. Such a sequence of recording steps consumes a considerable period of time. It is to be noted that a copy with black and magenta characters mixed together is produced by laying a black character document on a glass platen and, then, replacing it with a magenta character document.
Another prior art developing apparatus which belongs to a familiy of rotary multicolor developing apparatuses is constructed and arranged to inhibit a person from changing the order of arrangement of developing units (i.e., a yellow, a magenta, and a cyan developing unit); should a person try to relocate the cyan unit next to the yellow unit, the two units would interfere with each other to prevent such relocation. This means that the developing units are arranged in a fixed and unchangeable order.
A prior art rotary multicolor developing apparatus does not have a capability of supplying toner when toner in any of developing units becomes short (i.e., it is simply discarded once toner in any of the developing units is used up). This is undersirable from an economic viewpoint, because toner is not always consumed evenly in all the developing units. While one approach which may easily occur for the supply of toner is delivering toner of different colors to the respective developing units via a common toner delivery path, that approach would give rise to various secondary problems, such as mixing of toner of different colors and scattering of toner during delivery.
A rotary multicolor developing apparatus, as a matter of course, has a monocolor developing function for developing an image whose major part is occupied by black, red, blue, and other monochromatic lines, as is often the case with business copies, in addition to a full-color developing function adapted to develop a color photographic image and others. In a color electrophotographic copier with such a developing apparatus, there are selected latent those image developing conditions which are desirable in tone reproducibility so as to enhance the color balance of reproduced images. Major ones of such conditions may be the use of an electrically conductive carrier which exerts an insignificant edge effect, and the accommodation of great amounts of developers. A so conditioned color electrophotographic copier is capable of coping with considerable consumption of toner, which is inherent in color development, and enhancing reproducibility of photographic images.
Nevertheless, this type of prior art color electrophotographic copier is disadvantageous in that, since full-color development and monocolor development share the same developing conditions, designing those conditions with importance placed on color balance for full-color development would cause a reproduced image provided by monocolor development to appear excessively low in contrast, while designing them with importance placed on image quality attainable with monocolor development would render a reproduced image provided by full-color development excessively contrasty. One implementation which may be contemplated to solve such a problem is mounting a developing unit exclusive for monocolor (e.g. black-and-white) copying in a prior art color electrophotographic copier. This kind of scheme is not fully acceptable, however, because the extra developing unit limits the freedom of design of the other developing units, thereby decreasing their ability.